Composite structures formed of resin and fibers such as glass, aramid or carbon are used in many industries because of light weight and strength. A number of methods exist for forming composite structures. In one method, composite components are formed and pre-cured, then secured together. One method of securing the components is by the use of adhesive, which saves cost and weight as compared to drilling holes and mechanically attaching the components. Components may also be co-cured and bonded to each other at the same time, but co-curing requires tooling that holds the uncured components in position.
Process sensitivity and quality assurance issues have limited the use of adhesive bonding of pre-cured composites in some applications, such as in aerospace. These issues include sensitivity to surface preparation, bond line thickness, pressure distribution, moisture, and time-temperature curing profiles. In particular, the application of adhesive bonding on large composite structures has proven to be difficult for a number of reasons, as follows:
1. insuring uniform pressure application over large bonded areas;
2. insuring that no voids or bridged areas exist along the bond line;
3. demonstrating that variations in bond line thickness, which are characteristic of the interface between large structures, are acceptable; and
4. properly applying the adhesive on the large areas to be bonded in a timely and consistent manner to insure producibility from part to part.
One known technique uses a preform between the two components to be joined. A preform is a fabric member that may be woven or nonwoven. Normally, the preform is impregnated initially with a resin, but it will be in an uncured state. An adhesive film may be placed between the preform and each of the pre-cured components. Pressure and heat are applied to cure the preform and bond the components together. The pressure may be applied by vacuum bagging techniques. The preform has a thickness that causes it to compress under the pressure.